<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box,
        .item {
            border: 1px solid black;
        }

        .box {
            width: 500px;
            height: 250px;
            margin: 0 auto;
            padding: 10px;

            /* 直排弹性盒 */
            display: flex;
            flex-direction: column;

            /* 纵向（主轴方向）贴边 + 居中 + 均匀分布 */
            /* 1 # 1 # 1 # 1 */
            justify-content: space-evenly;


            /* 横向（交叉方向）贴边 + 居中 */
            /* align-items者 排列孩子也 */
            align-items: center;
        }

        .item {
            width: 100px;
            height: 50px;
        }

        .item1 {
            /* 当前孩子自己跑到交叉轴起始端 */
            align-self: flex-start;
        }

        .item3 {
            /* 当前孩子自己跑到交叉轴末端 */
            align-self: flex-end;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="item item1">1</div>
        <div class="item item2">2</div>
        <div class="item item3">3</div>
    </div>

</body>

</html>